It is well known that organic materials such as plastics, rubbers, lubricating oils, etc. are prone to oxidation and deterioration in the presence of oxygen. Oxidation of organic materials causes the loss of intrinsic properties which are characteristic of the organic material. It is also known that stabilizers useful in, for example poly-.alpha.-olefins, are not as effective or sometimes ineffective in the stabilization of diene containing elastomeric polymers. Further, the stabilization of diene containing polymers or rubbers is complicated by the fact that rubbers undergo Mooney viscosity changes and gel formation (among other modes of degradation) during the degradation process. With a view to preventing deterioration of rubbers, a variety of antioxidants have been developed. However, these antioxidants fail to completely prevent the deterioration of the desired physical properties of the rubbers to which they are added. Thus, those skilled in the art are constantly searching for new and more effective antioxidant systems which are useful for the protection of diene containing polymers or rubbers.
Antioxidant synergists have been known in the art for some time. For example, U.S. Pat. No. 3,492,336 discloses tetra-alkyl thioethyl thiodisuccinate compounds for use with phenolic type antioxidants in the stabilization of polyolefins. In addition, U.S. Pat. No. 3,758,549, discloses polyalkanol esters of alkylthio-alkanoic acids as synergists with phenolic antioxidants and U.S. Pat. Nos. 3,666,716 and 3,505,225 disclose derivatives of diphenylamines and the phenylnaphthylamines in combination with dialkyl 3,3'-thiodipropionates.
U.S. Pat. No. 3,010,937 is concerned with the stabilization of polyolefins such as polyethylene, polypropylene and polybutylene. This patent makes the statement that antioxidizing agents conventionally used for natural and synthetic rubbers are sometimes insufficient or are disadvantageous (for example, discoloration of the polymers or the formation of considerable odor) during the preparation of the polyolefins and the processing thereof. More specifically, U.S. Pat No. '937 discloses a process for stabilizing polymers of monoolefins against the influence of oxygen and ozone wherein the improvement comprises using thioacetals as stabilizing agents in an amount from 0.01 to 10% by weight based on the polymer. Specifically, this patent recites the use of formaldehyde-bis-dodecylthioacetals of the formula: EQU C.sub.12 H.sub.25 --S--CH.sub.2 --S--C.sub.12 H.sub.25
as stabilizers for polymers of mono-olefins, such as polyethylene. This patent does not suggest or disclose the instant invention which is concerned with the discovery that certain methylene bis(alkylsulfides), where the alkyl group can range from 6 to 14 carbon atoms, will dramatically increase the effectiveness of phenolic and amide antidegradants in the stabilization of diene containing polymers. Further, the U.S. Pat. No. '937 does not suggest or disclose a critical ratio between the methylene bis(alkylsulfides) and the antioxidant which would provide excellent antioxidative properties in addition to decreased gel formation and enhanced Mooney viscosity stability.
U.S. Pat. No. 3,258,449 is concerned with the stabilization of polyolefins through the use of 2,6-di-tertiary-butyl-p-cresol and organic sulfides. This patent claims that a compound having the formula: EQU R--S--(CH.sub.2).sub.n --S--R'
in which R and R' are hydrocarbon radicals containing 4 to 25 carbon atoms and n is an integer from of 1 to 4 and is useful in the stabilization of olefin polymers such as polypropylene. This patent does not suggest that specific methylene bis(alkylsulfides) when combined with di or tri-substituted phenolic antioxidants or amine antioxidants at specific ratios of antioxidant to methylene bis(alkylsulfides) would provide excellent antioxidative and anti-gelling properties to the elastomeric polymer in which it is incorporated.
U.S. Pat. No. 3,293,209 is concerned with the stabilization of low pressure solid alpha olefin polymers through the use of diethers of dimercaptans. This patent discloses an improved method of stabilizing low pressure solid .alpha.-olefin hydrocarbon polymers against process and oxidative degradation. More particularly, it relates to compounds such as diethers of dimercaptans which, in combination with phenolic antioxidants, stabilize the .alpha.-olefin polymers against process and oxidative degradation. The diethers of dimercaptans utilized in this patent correspond to the general formula RSR'SR wherein R is an alkyl group containing 12 to 20 carbon atoms and R' is selected from the group consisting of aralkylene, alicyclic, and alkylene alicyclic radicals. It is stated that the compounds described in this patent are themselves stabilizers for polyolefins and are particularly effective with phenolic antioxidants. This patent does not suggest or disclose the criticality of the methylene bis(alkylsulfides) alkyl radicals and the critical relationship of the antioxidant to the methylene bis(alkylsulfide).
U.S. Pat. No. 3,478,107 describes formaldehyde mercaptals and their use as antiwear additives in lubricating oil compositions. The formaldehyde mercaptals of the U.S. Pat. No. '107 are represented by the structure R--S--CH.sub.2 --S--R.sub.1 wherein R and R.sub.1 are each independently branched chain alkyl radicals having from 3 to 4 carbon atoms. This patent does not suggest or disclose the discovery of the instant invention wherein specific methylene bis thioethers can be used to synergistically enhance the antidegradative effectiveness of phenolic or amine antioxidants. Further, U.S. Pat. No. '107 does not suggest or disclose the discovery of the instant invention which unexpectedly reduces gel formation and changes in Mooney viscosity of the rubber.
U.S. Pat. No. 4,111,873 is concerned with the thermal stability of halogen containing polymers. Specifically, this patent relates to stabilizing halogen containing polymers, such as polyvinyl chloride, through the use of compounds such as 1,1 bis(octadecylthio)cyclohexane in combination with a divalent metal salt of a carboxylic acid. The U.S. Pat. No. '873 is specifically concerned with stabilizing homopolymers and copolymers of vinyl chloride, vinylidiene chloride or chlorinated polyolefins through the use of a compound such as 1,1-bis[(p-tert-butylphenyl)thio] cyclohexane in combination with a divalent metal salt selected from magnesium, calcium, barium or zinc stearate or mixtures thereof.
The art of combining an antioxidant with a synergist has already been put into practice in industry and such combinations are often highly effective. The inventor herein has prepared a variety of compounds which have been examined with a view towards obtaining more stable rubber compositions. As a result, it has been found that the combined use of certain methylene bis(alkylsulfides) with phenolic or amine antioxidants brings about an unexpectedly powerful antioxidative effect in addition to lessened gel formation and Mooney changes. None of the above cited patents or other literature has suggested the use of specific methylene bis(alkylsulfides) in rubber. Further, the prior art does not suggest the additional benefits that can be obtained when using specific methylene bis(alkylsulfides) with phenolic or amine antidegradants at specific ratios for the stabilization of rubbers.